How it's calculated
Wall area = Length × Height
Paintable area = Wall area − (Doors × 1.8 m²) − (Windows × 1.2 m²)
Paint litres = Paintable area × Coats ÷ Coverage per litre
"Length" can be the wall length (single wall) or the room perimeter (whole room). The result is rounded up to whole litres — round again to the nearest standard pack size (1L, 4L, 10L, 20L) when ordering.
Typical coverage rates
| Paint type | m² / L / coat |
|---|---|
| Interior emulsion (smooth wall) | 10 – 14 |
| Interior emulsion (textured wall) | 6 – 9 |
| Exterior emulsion | 8 – 11 |
| Enamel / oil-based | 12 – 16 |
| Primer | 10 – 12 |
FAQ
Should I include the ceiling?
If you're painting the ceiling, add its area (room length × width) to the wall total. Ceilings usually need 1 fewer coat than walls.
How accurate is this?
Within about ±10% for normal interior work. Texture, primer absorption, application method (brush vs roller vs spray), and operator skill all shift the actual usage.
Why round up litres?
Paint is sold in standard pack sizes (1, 4, 10, 20 L). Buying short forces you back to the shop mid-job and risks colour-batch differences.